solar constant The mean intensity of the solar beam in free space (i.e. before penetrating the Earth's atmosphere) at the average distance of the Earth from the Sun. The intensity is not strictly constant for all the wavelengths of radiation involved. The amount of variability is still a subject of debate, but is certainly very small apart from the long-term development in the history of the Sun.

solar constant Steady rate at which energy from the Sun is received from just outside the Earth's atmosphere. Its value is c.1.353 kilowatts per square metre (perpendicular to the Sun's rays).